This invention relates to a communication system between a base station and a base station controller in a mobile communication system and, in particular, to a communication system between a base station controller and a plurality of base stations serially connected to the base station controller by the use of a single shared communication line.
A mobile communication system is well known which comprises a base station controller connected to a mobile communication exchange through a wire link, and a plurality of base stations connected to the base station controller through wire links, respectively. Each base station is connected through a radio link to a plurality of mobile stations within a service area of the base station.
FIG. 1 is a block diagram showing an example of a structure of a typical conventional mobile communication system.
The mobile communication system exemplified in FIG. 1 includes a mobile communication exchange 600 connected through a communication line 630 to a public exchange network, a base station controller 610, and a plurality of base stations (#0, #1, #2) 620, 621, and 622. The mobile communication exchange 600 is connected through a communication line 640 to the base station controller 610. The base station controller 610 is connected to the base stations (#0, #1, #2) through independent communication lines 650, 651, and 652, respectively.
Mobile terminals have no direct concern with this invention and, therefore, are not illustrated in this figure and subsequent figures.
In the system illustrated in FIG. 1, the communication line 650 serves as a private communication line between the base station controller 610 and the base station (#0) 620. Likewise, the communication lines 651 and 652 serve as private communication lines independent from each other.
However, in case where a service area of a particular base station, for example, the base station (#0) 620 always has a small traffic (i.e., a load), the transmission efficiency of the communication line 650 is degraded.
In view of the above, development is made of a system in which a plurality of base stations having a small traffic are serially connected to a single communication line connected to a base station controller in order to share the single communication line among the base stations.
FIG. 2 is a block diagram showing an example of a structure of a conventional mobile communication system adopting a base station serial connection system of the type. A similar technique is disclosed in Japanese Unexamined Patent Publication (JP-A) No. 56-156034.
The mobile communication system exemplified in FIG. 2 includes a mobile communication exchange 700 connected through a communication line 730 to a public exchange network, a base station controller 710, and a plurality of base stations (#0 through #2) 720, 721, and 722. The mobile communication exchange 700 is connected through a communication line 740 to the base station controller 710. The base station controller 710 is connected to the base station (#0) 720 through a communication line 750. To the base station (#0) 720, the base station (#1) 721 is serially connected through a communication line 751. The base station controller 710 and the base station (#2) 722 are connected through a private communication line 752, in the manner similar to the conventional system illustrated in FIG. 1.
In the above-mentioned base station serial connection system, the communication line 750 is used as a shared communication line through which communication data between the base station controller 710 and the base station (#0) 720 and another communication data between the base station controller 710 and the base station (#1) 721 are multiplexed and transmitted, for example, according to a time-division multiplex communication system.
FIG. 3 is a view of a frame structure showing one example of the multiplex communication system for the shared communication line 750 in FIG. 2. In the illustrated example, a communication line of 2 Mbps is used. The communication line of 2 Mbps has a frame rate of 8000 frame/second. Each single frame is composed of 32 time slots each of which is eight bits long. Each time slot has a transmission capacity of 64 Kbps. In the frame structure illustrated in FIG. 3, the 32 time slots (TSO through TS31) contains a synchronization TS 800 for establishing frame synchronization and a control TS 810 for controlling the shared communication line. The remaining 30 time slots other than the synchronization TS 800 and the control TS 810 are used as TS groups 820 and 821 assigned to communication with the base station (#0) 720 and to communication with the base station (#1) 721, respectively. In this conventional example, the two base stations (#0) 720 and (#1) 721 are serially connected. In case where a greater number of base stations are serially connected, the 30 communication time slots are assigned to communication with the respective base stations, taking their traffics into consideration.
As described above, in the conventional mobile communication system, the transmission capacity of the shared communication line is preliminarily distributed fixedly to the respective base stations, taking their traffics into consideration, to realize serial connection of the base stations by the shared communication line.
However, the above-mentioned conventional base station serial connection system in the mobile communication system is disadvantageous in the following respects.
First, the maximum transmission rate to each base station is determined by the transmission capacity of the shared communication line, which is preliminarily physically assigned upon installation of the base station. In this connection, the transmission capacity must be assigned assuming the greatest traffic of each base station at the busiest state. Therefore, at the base station exhibiting wide variation in load, the transmission capacity assigned thereto can not effectively be utilized in a normal state so that the line efficiency is decreased. On the other hand, in case where an excessive load exceeding the maximum transmission rate assumed as mentioned above is temporarily applied, restriction is imposed upon transmission.
In case where a particular base station has a high load exceeding the maximum transmission rate, another base station may possibly have a small load leaving a spare transmission capacity. Even in this event, the spare transmission capacity can not be utilized by the particular base station of the high load because the transmission capacity assigned to each base station is fixed. Thus, restriction is imposed upon transmission so that the line efficiency of the shared communication line is decreased as a whole.
The transmission capacity of the shared communication line is preliminarily physically assigned to each base station upon installation of the base station. Therefore, in order to additionally install a new base station to be serially connected to the shared communication line, it is necessary to change the transmission capacity assigned to each of the existing base stations connected to the shared communication line. This requires modification in facility of the existing base stations.
In order to solve the above-mentioned problems, it is an object of this invention to provide a base station serial connection communication system in a mobile communication system, which can be flexibly adapted to load variation of each base station and additional installation of a new base station and which has a high line efficiency.
According to this invention, there is provided a base station serial connection communication system in a mobile communication system including a base station controller for controlling a plurality of base stations serially connected through a shared communication line, wherein:
a communication signal between the base station controller and each of the base stations serially connected is converted into a communication signal cell which is given header information containing an address identifier of a corresponding one of the base stations, the communication signal cell being collected into a cell multiplexed signal to be transmitted or received through the shared communication line.
The base station controller comprises:
a plurality of header addition/deletion units provided in one-to-one correspondence to the base stations serially connected to the shared communication line, each of the units being for converting a communication signal to be transmitted to a corresponding one of the base stations into a communication signal cell which is given header information containing an address identifier of the corresponding base station, and for deleting header information from a communication signal cell transmitted from the corresponding base station to convert the communication signal cell into a communication signal to be transmitted to a mobile communication exchange; and
a cell collection/distribution unit for collecting the communication signal cells obtained by conversion in the header addition/deletion units to produce a cell multiplexed signal to be transmitted through the shared communication line, and for distributing communication signal cells multiplexed in a cell multiplexed signal received through the shared communication line to corresponding ones of the header addition/deletion units in accordance with address identifiers contained in the header information.
Each of the base stations serially connected to the shared communication line comprises:
a header registering portion in which header information containing an address identifier of the base station in consideration is registered:
a cell multiplexing/demultiplexing portion which is for extracting, from a cell multiplexed signal delivered in a direction downstream from the base station controller through the shared communication line, a communication signal cell having the header information registered in the header registering portion and, if any base station is connected to a lower level downstream of the shared communication line, transferring the remaining part of the cell multiplexed signal towards a terminal end through the shared communication line and which is for collecting a communication signal cell to be delivered from the base station in consideration to the base station controller into a cell multiplexed signal to be delivered towards the base station controller through the shared communication line or, if any base station is connected to a lower level downstream of the shared communication line, multiplexing the communication signal cell onto a cell multiplexed signal supplied in a direction upstream from the terminal end; and
a header deletion/addition portion for deleting the header information from the communication signal cell extracted by the cell multiplexing/demultiplexing portion to reproduce a communication signal addressed to the base station in consideration and for adding the header information registered in the header registering portion to a communication signal to be transmitted from the base station in consideration to the base station controller to convert the communication signal into the communication signal cell to be transmitted to the base station controller.
A variable-length packet is used as the communication signal cell.
A fixed-length packet is used as the communication signal cell.
The shared communication line is a time-division multiplex communication line. The cell multiplexed signal is transmitted or received using as a user data area in a time-slotless fashion all of a plurality of time slots forming each frame of the time-division multiplex communication line except a synchronization time slot for establishing frame synchronization and a control time slot for use in line control.
The base station controller comprises:
an ATM (Asynchronous Transfer Mode) switch for switching an ATM cell;
a plurality of header addition/deletion units provided in one-to-one correspondence to a plurality of channel units each of which is for converting a reception signal supplied from a mobile communication exchange into a communication signal to be transmitted to a corresponding one of the base stations and for converting a communication signal supplied from each of the base stations into a signal to be transmitted to the mobile communication exchange, each of the header addition/deletion units being for converting the communication signal transferred from a corresponding one of the channel units into the ATM cell with header information added thereto and transferring the ATM cell to the ATM switch, the header information including an address identifier of a destination base station as a VCI (virtual channel identifier) and identifier information of a communication line connected to the destination base station as a VPI (virtual path identifier), the header addition/deletion unit being for deleting the header information from the ATM cell switched by the ATM switch to reproduce a communication signal which is transferred to a corresponding one of the channel units; and
a plurality of transmission units terminating the shared communication line and the private communication lines, respectively, for cell-multiplexing ATM cells switched by the ATM switch to produce a cell multiplexed signal to be transmitted to a corresponding one of the shared communication line and the private communication lines, and for transferring to the ATM switch the ATM cells multiplexed into a cell multiplexed signal supplied from a corresponding one of the shared communication line and the private communication lines;
the ATM switch being supplied with the ATM cell transferred from each of the header addition/deletion units for switching the ATM cell to a corresponding one of the transmission units with reference to the VPI, and being also supplied with the ATM cell transferred from each of the transmission units for switching the ATM cell to a corresponding one of the header addition/deletion units with reference to the VCI.
In this case, each of the base stations serially connected to the shared communication line comprises:
a header registering portion in which header information containing an address identifier of the base station in consideration is registered;
a cell multiplexing/demultiplexing portion which is for extracting, from a cell multiplexed signal delivered in a direction downstream from the base station controller through the shared communication line, an ATM cell having the header information registered in the header registering portion and, if any base station is connected to a lower level downstream of the shared communication line, transferring the remaining part of the cell multiplexed signal towards a terminal end through the shared communication line and which is for collecting an ATM cell to be delivered from the base station in consideration to the base station controller into a cell multiplexed signal to be delivered towards the base station controller through the shared communication line or, if any base station is connected to a lower level downstream of the shared communication line, multiplexing the ATM cell onto a cell multiplexed signal supplied in a direction upstream from the terminal end; and
a header deletion/addition portion for deleting the header information from the ATM cell extracted by the cell multiplexing/demultiplexing portion to reproduce a communication signal addressed to the base station in consideration and for adding the header information registered in the header registering portion to a communication signal to be transmitted from the base station in consideration to the base station controller to convert the communication signal into the ATM cell to be transmitted to the base station controller.
In this case, the transmission unit provided in the base station controller and the cell multiplexing/demultiplexing portion provided in each of the base stations serially connected through the shared communication line or individually connected through private communication lines insert into the cell multiplexed signal an ATM cell with a vacant cell header representative of inclusion of no communication signal in absence of a transmission ATM cell to be transmitted.
Thus, in the base station serial connection communication system in the mobile communication system according to this invention, adjacent base stations having a relatively small traffic can easily be serially connected by a cell multiplex transmission system, utilizing the existing facility and the existing communication lines of the conventional mobile communication system.
Particularly when a service area is set along a main road or a railroad line, it is possible to easily enlarge the service area by the use of a single physical communication line. As compared with star connection of base stations by the use of private communication lines, the line cost can considerably be reduced.
Furthermore, logical multiplexing is carried out by the use of the cell data which is given the header information containing the address identifier corresponding to each base station. This makes it possible to share the total communication capacity of the physical line (shared communication line) among all base stations serially connected to the physical line. Therefore, the load variation in each base station can be absorbed one another by the statistical multiplexing effect. Thus, occurrence of restricted communication can be considerably reduced to achieve an excellent line efficiency of the system.
In order to additionally install a new base station in a serial fashion, it is sufficient to assign a new address identifier to the new base station additionally installed. This is because the logical multiplexing is adopted. Since repeated modification of, for example, line setting of the existing base stations is unnecessary, it is easy to additionally install a new base station in a serial fashion.